1. Field of the Invention
The present invention relates to a power supply, and particularly to a resonant converter utilizing burst mode control.
2. Description of the Related Art
Current trends in power supply development include higher efficiencies, higher power densities and lower costs, in addition to, environmentally friendliness and energy efficiency. Thus, resonant converters are widely used in the power supply field due to its ability to be soft-switched and operated under a state of a maximum duty cycle so that highly efficiency can be achieved when a resonant converter has a heavy load. However, when a resonant converter has a light load, operating efficiency is not desirable.
Accordingly, the conventional resonant converter is operated in a burst mode to minimize the amount of switching operations and loss per unit time for higher operating efficiency. For example, as shown in FIG. 1, when the error amplifying signal Vea is equal to or greater than the upper threshold voltage Vref2 of the hysteresis comparator, the voltage-frequency converter is enabled to generate an oscillation signal during the burst mode working period, such that the switching elements of the half-bridge converter are switched according to control signals (LVG and HVG). On the contrary, when the error amplifying signal Vea is less than the upper threshold voltage Vref1 of the hysteresis comparator, the voltage-frequency converter is disabled to stop generating the oscillation signal, such that there is no driving signal for the switching elements of the half-bridge converter.
Despite, improvements may still need to be made to the described burst mode control method for the resonant converter. For example, because the error amplifying signal Vea may fluctuate between the upper threshold voltage Vref2 and the lower threshold voltage Vref1 and the error amplifying signal Vea is inversely proportional to the frequency of the oscillation signal fosc, the frequency of the oscillation signal fosc, in a single burst mode working period (BMWP), is increased when the error amplifying signal Vea is reduced. In addition, during the first few driving pulse periods of the single BMWP, a large resonant current is appeared due to the reducing of resonant impedance (i.e. resonant current unbalance), so that causes many problems such as large output voltage ripple, large audio noise, optimum operation point variation (magnetic bias of magnetizing inductor current and failure of zero-voltage switching), and so on.